Phase noise in oscillators, for example voltage-controlled oscillators (VCOs), is one of the most critical parameters in communications systems, particularly in wireless systems. Low phase noise VCOs bring a number of advantages to the system performance.
VCOs which are fully integrated on a semiconductor integrated circuit (IC) are today feasible for many communication systems. One is constantly striving for lower phase noise, since it can give further benefits to system performance. There is also a trade-off between phase noise and power consumption, which means that if phase noise is optimized, some of the phase noise performance may be traded-off for lower power consumption.
Phase noise in oscillators is strongly dependent on the signal amplitude in the oscillator. There is often a very distinct optimum amplitude where the phase noise has a minimum. Since this optimum amplitude may vary with tuning voltage (frequency), temperature, process (component) variations, ageing, etc., one solution is to try to integrate some form of automated amplitude regulation circuitry together with the VCO. It would be advantageous to measure phase noise and regulate the amplitude directly dependent on said measured phase noise.
Today, phase noise is measured with dedicated phase noise measurement systems. A new technique, based on locking two similar oscillators to each other, has recently been proposed in the paper “Phase-Noise Measurement Using Two Inter-Injection-Locked Microwave Oscillators”, M. Nick, A. Banai, and F. Farzaneh, IEEE Trans. Microw. Theory Tech., Vol. 54, No. 7, pp. 2993-3000, July 2006.
This proposed technique is shown in prior art FIG. 1, where two oscillators P1, P2 operating at similar frequencies are inter-injection locked to each other by means of a first directional coupler P3, a second directional coupler P4 and a phase shifter P5 via an attenuator P6. The outputs of the locked oscillators P1, P2 are mixed together in a mixer P7 after another phase shift, in another phase shifter P8, on one of the outputs, creating a quadrature input to the mixer P7. The signal output from the mixer P7 is low pass filtered in a low-pass filter P9, amplified in a low noise amplifier P10 (LNA), digitized in an analog-to-digital converter P11 (ADC) and then Fourier transformed into the frequency domain by a fast Fourier transformer P12 (FFT), here illustrated as a computer (PC).
However, this measurement method makes regulation based on phase noise measurement difficult, since the directional couplers and the phase shifters are components that are impractical to integrate into the design, resulting in that those parts have to be discrete parts. This does not provide a practical regulation method that is directly dependent on the phase noise due to its bulkiness.
Therefore, an amplitude regulating circuitry according to prior art has to be based on some other parameter than phase noise that can be detected on-chip. Typically one tries to keep the amplitude itself constant (over temperature, tuning, etc.) to a preset value.
Such an amplitude regulating circuit does not give an optimum phase noise performance over all conditions, since the optimum amplitude varies. It is also difficult to measure the amplitude with sufficient accuracy without disturbing the VCO itself.
A further complication is that the phase noise often decreases fairly slowly with increasing amplitude, and then reaches a minimum. Hereafter it increases at a fast rate if the amplitude is further increased. In order to ensure that the oscillator does not enter into the region where phase noise increases rapidly, one has to back-off to a lower than optimum amplitude to have some margin for process variations, frequency variations, etc.
If regulation is not considered as necessary, it is nevertheless desired to have easily accessible phase noise measurement possibilities, preferably integrated in the design. Existing phase noise measurement equipment is, however, expensive, fairly slow, and very difficult to implement in production testing of integrated VCOs.
Therefore, phase noise is very seldom tested in production at all. Instead it is common to only test DC current consumption and from that result draw indirect conclusions about the phase noise performance of the VCO. This can mean that some VCOs with bad phase noise slip through testing and are mounted in products. Either the problem is detected in system tests, or it is not detected until the equipment is at the customer. In both cases the bad VCO causes very large costs.
To minimize this problem, test limits, on e.g. DC current consumption, are set very conservative. Therefore a lot of VCOs may unnecessarily be rejected, which results in large extra costs.
There is therefore a demand for a VCO where the signal performance with respect to phase noise is optimized using enhanced measurement equipment which is possible to use for each VCO in production, as well as for phase noise regulation purposes.